1. Field of the Invention
This invention relates generally to processing of computer graphics for display on a television, and more particularly, to aperture correction for computer graphics.
2. Description of the Related Art
As the result of the continuous development of new technologies, the distinction between computers, in particular computer monitors, and televisions is becoming increasingly blurred. In other words, the computer and television industries are converging. For example, computer networks such as the Internet and the World Wide Web used to be almost exclusively a computer phenomena. Now, however, televisions may also be used to access these networks. As another example, broadcast entertainment used to belong squarely in the television domain. Now, however, many service providers are offering entertainment to computer users through computer networks. As a result of this convergence, there is a need to display computer graphics originally intended for computers on televisions.
The Need for Flicker Filtering
Televisions and computers, however, generally use incompatible graphics formats. For example, many formats for computer monitors and flat-panel displays are non-interlaced. In other words, the entire frame of computer graphics is updated at once. In contrast, many common television formats are interlaced, meaning that the frame is divided into odd and even fields and only one field or half the frame is updated at a time. As a result, in order to display computer graphics on a television, the computer graphics often must be converted from a non-interlaced to an interlaced format. This conversion typically includes dropping lines of the display. However, this introduces undesirable visual effects as a result of the conversion from a non-interlaced to an interlaced format.
In addition to the conversion process, typical systems also perform flicker filtering to improve the image quality. Two common types of flicker filtering are 2-tap and 3-tap filtering, in which either two or three non-interlaced lines are combined to form each interlaced line. Some attempts have been made to use vertical filters to limit the frequencies that will cause flicker when the progressively scanned (non-interlaced) image is converted to the interlaced format. Increasing the number of taps in the filters provides better frequency selection in eliminating the frequency components that cause flicker. However, having a large number of taps increases the complexity and cost of the flicker-filtering circuit. Additionally, although flicker-reduction filtering effectively improves flicker artifacts, it can also reduce vertical picture resolution if the filter implemented does not have sharp transfer characteristics. In order to prevent flicker on television displays, the computer graphics picture must be properly band limited in the vertical direction as to obey the Nyquist limit. This reduction of resolution is most apparent in text since text is likely to have high vertical frequency detail.
The Need for Picture Aperture Correction
Graphic accelerators that implement flicker filter reduction for the conversion of computer desktop graphics into television signals are likely to use simple vertical filter structures due to integrated circuit area and resource limitations. In order to overcome the lack of resolution after flicker-filtering, a system has been developed as described in U.S. Pat. No. 5,910,820, entitled "Correction of Flicker Associated With Non-Interlaced-To-Interlaced Video Conversion", hereby incorporated by reference, to improve text and other picture detail. This system improves picture detail by using aperture correction for enhancing the contrast of the picture. Aperture correction refers to the process of improving picture quality by changing the relative signal levels of the luminance (Y) signal or chrominance (C) signals derived from an original RGB (red, green, blue) encoded pixel. Luminance (Y) is a measure of the brightness of a pixel. Chrominance (C) is a measure of the color information of a pixel.
As described in the aforementioned patent application, aperture correction is based on three threshold values:
ACh: High threshold value PA0 ACm: Medium range threshold value PA0 ACI: Low range threshold value
There are three luminance regions defined in the range of values. Aperture correction increases the values of all inputs in the upper region, decreases the values in the lower region, and changes the values in the middle region, based on the following algorithm:
if Y&gt;=ACh then Yout Y+AC else PA1 if Y&lt;ACh and Y&gt;=ACm then Yout Y-AC else PA1 if Y&lt;ACm and Y&gt;=ACI then Yout=Y+AC else PA1 if Y&lt;ACI then Yout=Y-AC
For inverse aperture correction, the value of AC becomes negative.
The purpose of using aperture correction on images is to improve the appearance of positive-going high contrast images, such as black text on a white background. The reverse process is termed inverse aperture correction, and its purpose is to improve the appearance of negative-going high contrast images, such as white text on a black background. The word improvement in this context means a better picture as perceived by a typical observer. Typically, aperture correction works as any other global luminance enhancement mechanism and once the parameters are set, they cannot be changed on a pixel-by-pixel basis.
The value to be added or subtracted from the input luminance Y signal is labeled AC and it can be any quantity in the range of values. However, to improve the signal, this value must be carefully selected in order not to affect, degrade or totally obliterate the input. Otherwise, the result of the current process is an image which appears slightly unnatural, because not only are images which are supposed to have high contrast highly contrasted, images that are more muted in contrast will appear highly contrasted. A user could manually adjust the amount of contrast for each frame, but that reduces the commercially practical usefulness of using aperture correction, because only users knowledgeable about the compromises and consequences of setting the parameters to the correct values would be able to use the device. Additionally, a user can spend a considerable amount of time trying to find the optimum values for a class of images only to find out that the parameters do not hold with other types of material. Thus, the user is forced to constantly readjust the circuit. Thus, there is a need for a system which can dynamically and adaptively add aperture correction only to those parts of an image frame that need it, specifically, the parts of the image containing text.